This invention relates generally to inlets for gas chromatographs and more particularly to an inlet splitter apparatus and method for a capillary gas chromatograph whereby substantially linear fractionation of intermixed sample and carrier gases is achieved to provide pure or substantially unreacted sample aliquots for chromatograph analysis.
Gas chromatography (more precisely gas-liquid chromatography) is a process by which a mixture is separated into its constituents by a moving gas phase passing over a nonvolatile liquid sorbent coated on an inert solid support. The basic apparatus comprises a column support for the sorbent, a carrier gas supply and control, a sample port or inlet, and a detector for determining the composition of the moving gas effluent from the column. In general, gas-liquid chromatographic columns are of two types: the packed column in which the liquid sorbent is distributed as a thin film on a granular support packed into the column, and the open tubular capillary column in which the interior of the capillary tube is coated with a thin layer of the liquid sorbent. The capillary columns are further classified as small-bore (i.e., about 0.25 mm) and wide-bore (i.e., about 0.75 mm). Since small-bore capillary columns can be demonstrated to give vastly superior resolution as respects the constituent components, their use is fast increasing. However, because of their small size, small-bore capillary columns present a number of problems. One of these problems is the greatly reduced sample capacity of the small-bore column, which limits sample injections to amounts of the order of 0.01 microliter. Another problem arises from the fact that small-bore capillaries utilize relatively low flow rates of the order of 0.5 to 1.5 milliliters per minute, with the result that column efficiencies will decrease quite rapidly as the average linear velocity in the column departs from an optimum value (usually within the range from about 13 to 25 centimeters per second). Another problem arises because the volume of the inlet port for the sample must be quite small in relation to the volume of carrier gas flow through the system since, otherwise, an effect known as "band broadening" is experienced with disastrous results as respects constituent resolution. While this band broadening effect is not a particular problem with packed or wide-bore capillary columns (since the high flow rates of carrier gas flush the inlet chamber clean in a very short time interval), the relatively low flow ragtes encountered in capillary columns produce such severe band broadening effects as to render conventional inlet ports entirely unsatisfactory.
In an effort to counter certain of the foregoing deficiencies, it has become common in the operation of small-bore capillary columns to employ an inlet splitter. In general, the inlet splitter functions to vaporize the small quantity of sample in a much greater volume of carrier gas and to pass the vapor-gas mixture over the capillary column inlet so that only a small aliquot is introduced onto the column whereas the major portion of the mixture is vented to the atmosphere. In general, to insure good resolution in the chromatograph, it is essential that the injected sample be vaporized instantly and completely to the carrier gas, that no residual sample remains in the inlet chamber, that the vaporized sample be completely introduced on the shortest possible segment of the column, and that the sample be followed by pure carrier gas rather than carrier gas containing exponentially diluted sample. It is also highly desirable that the split to the column be essentially linear, to insure that the concentration of each component split to the column is a function only of its concentration in the injected sample.
In practice, reliable and accurate isolation of the split sample is not always effectively accomplished. Thus, it is a common experience of workers in this field that many compounds of interest in the original material are relatively labile with the result that rearrangement and degradation of the constituents within the chromatograph frequently result. It can also be demonstrated that the fractionation of the sample is not truly linear, and that the portion entering the capillary column is not identical in composition with the original bulk sample. Causes of these difficulties are many, including changes associated with rapid vaporization of the samples in the presence of hot metal surfaces, and the use of inexact splitting arrangements such as simple concentric tube splitters. As a consequence, despite the common use of inlet splitters in conjunction with the capillary chromatographic columns, the development of a truly reliable linear inlet splitter, by which substantially pure aliquot samples can be introduced to the capillary tube, is highly to be desired.